The invention described herein as follows is concerned with a significant improvement in the aqueous ethanol extraction process for making soy protein concentrates. For the production of these concentrates, it is customary to employ clean soybeans of either a selected variety or field-run origin. These are dried, cracked, dehulled, moisture conditioned, and flaked prior to solvent extraction to remove the vegetable oil. The solvent is a selected fraction of lower alkane hydrocarbons known commercially as "hexane." After extraction the solvent-laden flakes are thoroughly desolventized and dried by heating with direct and then indirect steam. The preparation, extraction, desolventizing and drying of the soybean flakes are carried out in various continuous operating devices under a variety of moisture and temperature conditions. Although such devices and conditions are not a part of the present invention, they are important in establishing the physical and chemical nature of the defatted flakes which are used in the aqueous alcohol process for the production of soy protein concentrates. The thickness of such flakes is established by the spacing of the cracked bean flaking rolls prior to the solvent extraction unit operation. These rolls are set to produce full-fat flakes having a thickness of about 0.010 to 0.020 of an inch. For aqueous alcohol concentrate production, the desolventizing and drying operations are carried out under conditions which cause a minimum of protein denaturation in the defatted flakes. Excessive denaturation (e.g., toasting) results in a reduced rate of extraction of the soluble components by the aqueous alcohol.
The mechanical handling, conveying and transportation of defatted soybean flakes results in much breakage due to their fragility. The breakage produces fines or dust, very small particles in a range of sizes. The defatted flakes have a porous nature because of removal of oil. This enhances fragility. Fragility is related to flake thickness, and importantly, to the amount of moist steaming and heating they receive. This treatment results in protein denaturation with toughening of the flake. As noted above, this treatment is controlled so as not to adversely affect the aqueous alcohol extraction rate. The extent of such treatment is ordinarily determined by measuring the amount of water-soluble nitrogen or protein in the flake. One method is the Nitrogen Solubility Index or NSI (Method Ba-11-65, Official Method of the Am. Oil Chem. Soc.). Flakes ranging in NSI from about 50 to about 80 are usually used in the aqueous alcohol process. Flake fragility is related, in part, to NSI in that flakes with higher values are more fragile. Flakes with very low NSI, below 20, are most often avoided since excessive denaturation results in slower diffusion of the extractable flake constituents which include sugars, nitrogenous substances, mineral matter, etc. Obviously, the most significant factor in the production of flake fines and dust is the nature and extent of handling and transportation. Defatted flakes contain 10 to 30 percent material passing a No. 25 U.S.A. Standard Sieve (0.0278 in. nominal opening). In some instances, the content of such fine material may reach as high as 50 percent.
A variety of continuous countercurrent (flake to solvent flow) extraction devices of varying design are commercially available. Although these devices have been designed for the hexane extraction of vegetable oils from various seed products, several of these have been adapted for the production of soy protein concentrate.
Two basic principles are utilized in the design of the countercurrent hexane extractors. One is the immersion (or submergence) principle wherein the fat-bearing material (flake) is totally immersed in solvent and moves in a direction opposite to the solvent flow so that in-coming seed material is contacted with an oil-rich solvent or miscella, and the exiting material or flakes are contacted with fresh solvent, thereby insuring efficient removal of oil or fat. In devices operating according to the percolation principle, the fat-bearing material is automatically and continuously fed to moving containers or compartments with perforated bottoms. The drained solvent phase is pumped from one compartment to another countercurrent to the movement of fat-bearing seed material. Both horizontal and vertical devices are employed to remove the oil from full-fat soybean flakes by percolation of solvent phase through successive beds of said flakes with the oil-rich solvent phase percolating through the in-coming flakes and fresh solvent contacting the exiting defatted flakes. For a more thorough discussion of countercurrent extraction devices, see (a) H. D. Fincher, "Processing of Oilseeds" in Processed Plant Protein Foodstuffs, A. M. Altschul, Editor, Academic Press, 1958, (b) A. Garcia Serrato, "Extraction of Oil from Soybeans," J. Am. Oil Chem. Soc., 58, 157 (1981), and (c) E. D. Milligan, "Survey of Current Solvent Extraction Equipment," J. Am. Oil Chem. Soc., 53, 286 (1976).
In the utilization of these countercurrent devices for the aqueous alcohol extraction of defatted soybean flakes containing fines or dust, a number of serious processing problems arise which are proportional in magnitude to the amount of fines (e.g. minus No. 25 U.S.A. Standard Sieve) and particularly, the preponderance of very fine particles. In both immersion and percolation extraction systems, the fines and dust get into the solute-containing solvent stream. For example, in a vertical immersion column, the dust and small particulates tend to become entrained because of the greater density of the aqueous alcohol as compared to hexane (0.89-0.93 g/ml vs. 0.65-0.67 g/ml. for hexane; water about 1.0 g/ml). The floating dust leaves the extractor in the solute-rich solvent stream. In percolation extractors, the dust passes the perforations in the bottom of the compartments. Again, such dust is entrained in the exiting solvent stream. The fines and dust swell in the aqueous organic solvent and cause serious fouling and even plugging of pipes, circulation pumps, control valves, flow meters, and the like. This particulate matter settles in holding tanks causing additional problems. Obviously, the recovery of alcohol is a very vital factor in the economics of the production operation. Evaporators are customarily used for such recovery resulting in a soy sugar syrup byproduct. During such a process the fines and dust cake on the internal walls and components of the evaporators, impeding heat transfer and eventually necessitating a shutdown and cleaning. Since the unit is out of service during such cleaning, it impacts on product throughput and hence economy. It should be noted that fines and dust also cause foaming of the miscella during evaporation resulting in "spill-over" with contamination of the purified solvent. Antifoams must be used to prevent this insofar as possible.
Swollen fines cause another serious problem in percolation extractors. Such fines plug the drainage perforations impeding the flow of extraction solvent through the flake bed. In order to alleviate this problem, the bed depth is lowered to less than optimum, thereby restricting the throughput of the extractor. When vertical tray-containing or plate extractors are shut down for servicing of auxiliary equipment, the fines settle in the bottom trays impeding smooth startup.
Fines and dust in the solvent-wet concentrate cause further problems in the desolventizing-drying equipment. Poor drainage results in increased solvent entrainment and high desolventizing loads. The fines cake on heated surfaces resulting in less efficient heat transfer. The dust also plugs wet cyclones, condensers, and vapor pipes. It may even enter the air exhaust system, clogging filters.
It is quite apparent that fines and dust in the flake feed stream for the production of soy protein concentrate by aqueous alcohol extraction cause problems which are manifold and quite serious. It has been proposed that the problem be resolved by screening the flake stream just prior to extraction. This is an unsatisfactory resolution of the problem since it generates a significant by-product stream which must be disposed of at a reasonable price to maintain an economic processing operation. Further, such a stream will vary in volume with changing flake lots, making the problem of uniform disposal all the more difficult.